Composition useful in transparent conductive coatings

ABSTRACT

Provided is a conductive coating composition which comprises a colloidal dispersion of cuprous iodide particles in a binder resin solution which contains an ohmic bridging electrolyte. The preferred ohmic bridging electrolyte is potassium iodide. The colloidal dispersion also preferably exists in an aqueous system, with the binder resin being a water soluble polymer such as a polyvinyl alcohol. Upon coating a support with the dispersion, a transparent conductive coating is achieved comprised of ohmic bridged cuprous iodide particles and the binder resin. Such a coating composition can be advantageously used in the manufacture of imaging elements, and provides many advantages in the handling and manufacture of the imaging elements, as well as in the adhesive properties exhibited by the imaging elements.

BACKGROUND OF THE INVENTION

The production of unitary conductive elements which are particularlyuseful in such areas as electrophotography and electrostatic films andpapers in general, have been extensively described in patents and otherliterature. Many of these conductive elements have multilayer structuresand are prepared by coating a substrate layer with a conductivematerial. A further coating may then be added, as for example, in anelectrophotographic element, wherein a layer of photoconductivecomposition is coated over the conductive material. If desired, abarrier layer may be imposed between the conducting material and thephotoconductive compositional layer.

One of the many problems encountered in the process of producingconductive elements, and particularly those useful in electrophotographyand electrography, is that there is a difficulty in obtaining goodadhesion between the various layers. Moreover, the uniform dispersionsof the conducting material used in producing the conductive elementsusually include a polymeric binder. Solvents for the polymeric binderand the conductivity material are often difficult to obtain sinceconducting materials are often insoluble in the polymer solvent and viceversa.

The foregoing disadvantages for producing conductive elements, and inparticular electrophotographic elements, are even more problematic whenthe conductive layer is a cuprous iodide (CuI) coating. For instance, inU.S. Pat. No. 3,245,833, a volatile organic solvent is used tosolubilize the binding material and to dissolve the solubilizedsemiconductor compound. But in order to solubilize the semiconductorcompound a complexing agent must also be added. This complexing agent isusually a chelating agent, and in some cases it can be the solvent.After the coating of the solution of CuI, the solvent is then evaporatedand CuI particles are formed in-situ in the coating after drying.

U.S. Pat. Nos. 3,597,272 and 3,740,217 suggest another method ofachieving, specifically, an electrophotographic element while overcomingthe problems of layer adhesion and mutual solvents. An imbibitionprocedure is disclosed. The conductive layer is formed by imbibing abinder-free solution of volatile solvent and a metal-containingsemiconductor into an electrically insulating polymeric subcoatingcarried on a support, and then evaporating the solvent. Many of theexamples illustrate the use of a solution of cuprous iodide inacetonitrile as the volatile solvent.

The use of acetonitrile as a solvent for the coating process of CuI iswell known. However, when using acetonitrile, the uniformity of theconductive coating is difficult to control. East German Patent Nos.DD223,550, DD220,155, DD201,527, DD157,369, DD157,368, and DD149,721illustrate the preparation of conductive layers containing CuI fromorganic solutions, including acetonitrile solutions, or the preparationof opaque conductive stripes for the purposes of annotation using adispersion of CuI in a binder. The use of acetonitrile solutions canalso be hazardous. For example, acetonitrile is known to produce HCNunder thermal degradation at high temperatures.

While environmental impact and safety concerns are items that must beconsidered whenever organic solvents are used in coating processes, theprior art processes are fraught with hazards due to the use orgeneration of toxic materials during the processes. Numerous patentsexist which describe processes utilizing a solution approach to thedeposition of CuI from highly toxic solvents, such as, for example,acetonitrile. See, for example, U.S. Pat. No. 3,505,131. Fuji Japanesepatent publication 58/136044 discloses a binderless coating processwhich utilizes acrylonitrile solvents. Thereby, highly toxic solventsare again resorted to in the manufacture of films, the resistivity ofwhich is believed too low, in any event, for electrostatic imagingmaterials.

V. Sumita et al disclose in the Journal of Appl. Polymer Science, Vol.23, pp. 2279-91 (1979), a process for making conductive films bytreating Cu complexes of divalent copper and polyvinyl alcohol withvapors of iodide carried in acetone. The entire procedure requiresspecial measures to contain the highly toxic iodine vapors and extremelyflammable acetone vapors. There are also side reactions which render thesystem non-conductive and make the process unstable.

It would therefore be advantageous to be able to utilize a materialuseful in transparent, conductive coatings which exhibits good adhesionproperties, and which also permits its manufacture and handling withoutexposure to toxic chemicals.

It is therefore an object of the present invention to provide atransparent conductive coating dispersion material which is devoid of atoxic solvent and which is suitable for use in manufacturingelectrostatic and electrophotographic films.

Another object of the present invention is to provide such a materialwhich when coated onto a substrate forms a conductive layer whichexhibits good adhesion to the substrate.

It is another object of the present invention to provide such a materialthat forms a conductive layer which also exhibits good adhesionproperties with respect to any dielectric or photoconductive layers.

Yet another object of the present invention is to provide a process forpreparing a ground plane suitable for use in electrostatic andelectrophotographic films, as well as antistatic materials, which avoidsthe use of toxic materials and avoids any residual harmful or toxicsolvents in the conductive layer.

It is another object of the present invention to provide such a groundplane which is humidity independent.

Still another object of the present invention is to provide such aground plane having a conductive layer which exhibits excellenttransparency in the visible spectrum and the UV spectrum suitable fordiazo reprographic processes.

These and other objects of the present invention will become apparentupon a review of the following specification and the claims appendedthereto.

SUMMARY OF THE INVENTION

In accordance with the foregoing objectives, there is provided by thepresent invention a conductive colloidal dispersion of cuprous iodideparticles in a binder resin solution which contains an ohmic bridgingelectrolyte. Such a coating composition is, in general, useful as anovel coating in the preparation of a ground plane forelectrophotographic and electrostatic imaging elements, as well asantistatic materials. In a preferred embodiment, the ohmic bridgingelectrolyte is potassium iodide or sodium thiosulfate. Upon coating asupport with the dispersion a conductive coating is achieved comprisedof ohmic bridged cuprous iodide particles and a binder resin. The ohmicbridges between the cuprous iodide particles are established by theohmic bridging electrolyte.

It is most preferred that the colloidal dispersion exists in an aqueoussystem, with the binder resin being a water soluble polymer such aspolyvinyl alcohol. In a most preferred embodiment, a silanol modifiedpolyvinyl alcohol is added to the binder resin solution as a dispersingand milling aid.

A process for the production of such a coating material and themanufacture of imaging elements utilizing the novel coating material arealso provided herewith.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The novel transparent conductive coating composition of the presentinvention comprises a dispersion of cuprous iodide particles and anohmic bridging electrolyte in a resin binder solution. The dispersion ispreferably of colloidal cuprous iodide particles. It is understood inthe art that a colloidal dispersion is a suspension of finely dividedparticles in a continuous medium. The particles themselves are calledthe disperse phase, or the colloid, and the medium is the dispersingmedium. The colloidal dispersion differs from an ordinary solution ordispersion in that the size of the particle lies in the range between 1and 0.001 micron. While it is preferred that the dispersion of thepresent invention contain only particles of submicron size, it isintended for the purposes of the present invention to cover thoseinstances where the dispersion does contain a relatively small amount ofparticles having a size greater than 1 micron, and which particlesnormally would not be considered "colloidal" in nature.

In the present invention it is preferred that the average particle sizeof the cuprous iodide particles is about 300 nanometers or less, andmore preferably is about 200 nanometers or less. The most preferredaverage particle size of the cuprous iodide particles is in the range offrom about 90-170 nanometers. Such a particle size results in a visualbackground density of about 0.1 and an absorption density below 0.2 inthe UV spectrum. The dispersion is also most preferably an aqueoussystem, with the binder resin being a water soluble polymer such aspolyvinyl alcohol.

It has been found that the presence of the ohmic bridging electrolyte inthe coating composition of the present invention provides manyadvantages. For example, when the concentration of the binder is high ina conductive coating composition, the conductive particles could beseparated from one another due to the formation of a polymeric film onthe particles by the resin binder. This formation of a film could renderthe coating insufficiently conductive for its intended purpose, or atleast less conductive than expected or desired. This phenomenon couldhappen at the minimal binder concentration required and/or desired foradhesion and grinding purposes. It has been surprisingly found that thepresence of small amounts of an ohmic bridging electrolyte in thecoating composition avoids this problem and provides for uninterruptedconductivity in the resulting conductive layer. As a result, regulationof the required conductivity for electrographic, electrostatic andantistatic materials is much easier. Upon coating the composition onto asupport, it is believed the ohmic bridging electrolyte permits theconductivity between the cuprous iodide particles to be uninterrupted byforming ohmic bridges between the cuprous iodide particles.

In addition to the advantages of improved and more predictable andreproducible conductivity, the presence of an iodide electrolyte, and inparticular potassium iodide, also advantageously results in the removalof free iodine from the coating dispersion. This removal is believed tobe achieved in two different ways. First, the electrolyte itself isbelieved to complex with the free iodine. Second, the electrolyte isbelieved to act as a catalyst for the free iodine to complex with thepolyvinyl alcohol. The removal of the free iodine results in a coatingof improved transparency and color, as any yellow background color iseliminated.

It is also believed that iodide electrolytes, and in particularpotassium iodide, prevent contamination of the cuprous iodide particlesfrom contaminants introduced into the dispersion through contact withany equipment used in the preparation or coating of the dispersion.

The electrolyte can be any appropriate electrolyte for forming ohmicbridges between the cuprous iodide particles, and are preferably alkalimetal iodides, thiosulfates, bisulfites and dithionates. In particular,the iodides, thiosulfates, dithionates and bisulfites of sodium andpotassium are preferred, with the iodides, and in particular potassiumiodide, being most preferred.

The amount of ohmic bridging electrolyte employed in the practice of thepresent invention can vary based upon the amount of cuprous iodideemployed in the dispersion. In general only a small amount of theelectrolyte is needed. Based upon the weight of the conductive coatingcomposition, the amount of electrolyte employed is generally in therange of from about 0.5 weight percent to about 3 weight percent basedon a dry coating, and most preferably in the range of from about 1.0weight percent to about 2 weight percent based on a dry coating. In thedispersion, the amount of electrolyte added will generally range fromabout 0.05 wt % to 0.3 wt %.

The amount of cuprous iodide employed in the dispersion will generallydepend upon the desired application of the ultimate article to bemanufactured, which dictates the surface resistivity needed for theconductive coating layer. In general, such transparent conductivecoatings preferably have a surface resistivity of from about 10⁴ to 10⁹ohms per square, with the surface resistivity being modified by changingthe pigment (cuprous iodide) to binder ratio. When an electrostaticmaterial is to be manufactured, the surface resistivity is preferably inthe range of from about 0.5 to 5×10⁶ ohms per square. When anelectrophotographic material is to be manufactured, the surfaceresistivity of the conductive layer is preferably below 10⁶ ohms persquare. The cuprous iodide to binder ratio employed in the coatingcomposition is accordingly adjusted.

The term "surface resistivity" generally refers to the measurement ofelectrical leakage across an insulating surface. However, in the presentspecification, the term is used with reference to the resistance orconductivity of films that behave as conductors transmitting currentsthrough the body of the coating of electrically conducting andsemiconducting materials. Moreover, in the case of thin conductivecoatings, measurement of the conductive property in terms of surfaceresistivity provides a value that is useful in measurement and practice.

A suitable binder resin for use in the present invention may be selectedfrom any of the water soluble binders. The most preferred binder is thatof polyvinyl alcohol. The polyvinyl alcohol can be fully or partiallyhydrolyzed polyvinyl alcohol, or a modified polyvinyl alcohol. Suchpolyvinyl alcohol binder can be used alone, or in various combinations.Other suitable water soluble binders include without limitation,methylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, solublegrades of gelatin, carboxylmethylcellulose, soluble grades ofstyrene-maleic anhydride copolymers, water soluble acrylic polymers andpolyvinylpyrrolidone.

Generally, from about 0.1 to 10 weight percent binder is utilized in thefinal dispersion of the coating composition of the present invention,with from about 0.2 to about 2 weight percent being most preferred. Oncethe dispersion is coated onto a suitable support and dried, the bindercontent in the coating layer will generally range from about 1 to 20 wt%, with from about 3 to 1? wt % being most preferred. The moreconductive one desires the coating, the less binder that is used. Thedesired conductivity, of course, will depend upon the ultimateapplication.

The binder is generally dissolved in an aqueous solvent system. It isthe use of this aqueous system which avoids the problems of handling andworking with toxic organic solvents as in the prior art.

In a most preferred embodiment, silanol modified polyvinyl alcohol isadded to the coating dispersion. The amount of silanol modifiedpolyvinyl alcohol added to the dispersion is generally in the range offrom about .0025% by weight to about 2% by weight, and most preferablyin the range of from about 0.15% by weight to about 0.5% by weight. Theinclusion of the silanol modified polyvinyl alcohol has been found to bevery advantageous for several reasons. First, its presence results indecreased foaming during the processing of the CuI dispersion, and henceless defects are observed in any subsequent coatings due to the absenceof air bubbles. The silanol modified polyvinyl alcohol also acts as agrinding aid, reducing the necessary time involved in obtaining a betterdistribution of particles. The wetting properties of polyester supportsare also not a problem when silanol modified polyvinyl alcohol ispresent, and greater water resistivity is exhibited by the resultingconductive coating. The presence of silanol modified polyvinyl alcoholalso stabilizes the CuI dispersions to help preclude excessive settlingbefore a colloidal state is achieved during grinding, and to helppreclude any agglomeration of the CuI particles once a colloidaldispersion is achieved. For the foregoing reasons, it is most preferredto include silanol modified polyvinyl alcohol in the coatingcomposition.

The colloidal dispersion is prepared by adding the above-identifiedingredients, i.e., the cuprous iodide, ohmic bridging electrolyte andaqueous binder resin solution, and milling these ingredients over aperiod of time, under ambient conditions. Any type of milling isgenerally appropriate, e.g., ball milling, sand milling or mediamilling. Media milling using horizontal media mills such as thosemanufactured by Netzsch are efficient and dispersions prepared using thehorizontal media mill have been found to produce excellent coatings. Thetransparency of the films that are formed by this method can also beaffected by the duration of milling.

In a most preferred embodiment, the colloidal dispersion is achieved bya combination of grinding and separation steps as described incopending, commonly owned application U.S. Ser. No. 603,002 filedconcurrently herewith. In the process, it is most preferred that atwo-step grinding process is utilized, with the initial grinding beingconducted in a media mill and the final comminution of the particlesbeing achieved in an impingement mill. A bi-modal distribution ofparticles is accordingly achieved, with the mode of smaller particlesbeing separated, preferably by centrifugation of filtration.

By using such a grinding/separation method in preparing the colloidaldispersion, a colloidal dispersion having an average particle size of300 nm or less, more preferably 200 nm or less, and most preferably CuIparticles having an average particle size in the range of from about 90to 170 nm, can be most efficiently and effectively achieved. Thecolloidal dispersion is also quite stable and the sizes of the particlesare within a narrow range. The use of such colloidal dispersions alsoresults in extremely transparent conductive coatings, with lighttransmission in the visible spectrum being as high as 80%, or even 90%.Transparency has also been observed in the diazo reprographic range ofthe UV spectrum.

After the colloidal dispersion has been milled and/or subjected to aseparation step, an electrostatic film, for example, can then be made bylayering the colloidal dispersion onto a support layer of choice, e.g.,by a Mayer rod. Any support or substrate layer may be utilized toproduce a ground plane for a conductive element such as anelectrophotographic imaging element. These supports generally consist ofpolymer films such as polyethylene terephthalate films (PET),polyethylene films, polypropylene films, bond-coated polyester films, aswell as any other support utilized in the art. Other supports, however,such as paper supports, can also be appropriately used. The supportmaterials may be properly selected according to the use and purpose ofthe electrostatic imaging element.

Furthermore, the present invention is not limited to any particularmeans for applying the colloidal dispersion onto the substrate orsupport layer and any suitable means may be used such as coating by aMayer rod, roll coating, gravure, offset gravure, whirl coating, dipcoating, spray coating, etc. The means for applying the colloidaldispersion is only limited by the fact that a colloid may be difficultto apply with various forms of instrumentation.

The preferred coating weight of the conductive colloidal dispersioncoating on the support, e.g., polyester support, is generally in therange of from about 0.1 to 0.5 g/m², and most preferably from about 0.25to about 0.35 g/m².

A further coating, e.g., of an electrostatic layer, may then be coatedon top of the conductive layer. For example, a polymeric support can becoated with a cuprous iodide colloidal dispersion layer in accordancewith the present invention, and then an electrostatic layer can be addedto complete the formation of an electrostatic film.

If an electrophotographic material is desired, a barrier layer may befurther coated over the conductive layer with any known coating that isavailable in the art, such as a polymer of polyvinyl alcohol orpolyamide. A photosensitive layer can then be added to complete theformation of an electrophotographic film.

The coating composition of the present invention results in transparent,conductive coatings which exhibit excellent adhesion to the support,e.g., a polyester substrate, on which it is coated, as well as superbcohesive forces. Thus, it is difficult, if not impossible, to lift anyof the coating from a support such as a polyester substrate even aftercutting the coating in a criss-cross pattern with a sharp knife. Theadhesion of a dielectric coating to a conductive ground plane preparedby coating a support with the composition of the present invention isalso excellent. No lift is observed when adhesive tape (Scotch.sup.®tape) is applied and then pulled off, nor when the dielectric coatedmaterial is submerged in boiling water for up to 5-10 minutes at least.

Thus, the present invention offers many advantages. Among the advantagesare that the colloidal dispersion is extremely stable and does notchange significantly in terms of the average particle size, binder stateor in overall physical chemical properties, thereby rendering thecolloidal dispersion extremely workable and advantageous in its use.Furthermore, one can avoid exposure to volatile and toxic solventsduring the manufacture of the colloidal dispersion composition of thepresent invention and during the manufacture of a conductive groundplane upon coating the colloidal dispersion on a suitable support. Usingconventional techniques, a truly thin film of about 0.15 microns can beeasily obtained. The conductive layer also contains no residual volatileor toxic solvents, and does not contain any surface active components,e.g., phosphates or succinates. The composition of the present inventionis also quite flexible in that clear and transparent films could be madeto satisfy the requirements for electrostatic, electrophotographic andantistatic materials by simply altering the ratio of the cuprous iodideto binder resin ratio in the composition, and by appropriately adjustingthe addition of the ohmic bridging electrolyte.

The invention will now be more fully explained by the followingexamples. However, the scope of the invention is not intended to belimited to these examples.

COMPARATIVE EXAMPLE 1

A colloidal dispersion was prepared by using a two-step grindingprocess.

845 g of technical grade CuI were dispersed in 1000 grams of 2% solutionof fully hydrolyzed polyvinyl alcohol (Airvol 107 from Air Products).The resulting dispersion was treated in a laboratory Eiger Mill for 2hours, until the average particle size of the dispersion was reduced toabout 1.2 microns. This dispersion was then subjected to 10 passesthrough an impingement mill (Microfluidizer Model M110Y fromMicrofluidics Inc.) until a bimodal distribution was achieved at anaverage particle size of 505 nanometers. The mode consisting of thesmaller particles was separated by centrifugation. The final dispersionhad an average particle size of 148 microns and had a yellow bluishtint.

The resulting dispersion was coated with a Mayer rod #2 onto a polyestersupport and dried, which gave a conductive coating having a surfaceresistivity of about 5×10⁸ ohms per square. When the dry coating wasexamined, a small amount of surface defects was found, due to a collapseof air bubbles dispersed in the wet coating. The resulting coating wastransparent in all ranges of the visible spectrum. Its lighttransmittance in the reprographic area of the ultraviolet spectrum wasabout 17%.

EXAMPLE 1

This example demonstrates the excellent coating effects achieved whensilanol modified polyvinyl alcohol is used in the coating composition. Acolloidal dispersion was prepared as in Comparative Example 1, but the1000 grams of polyvinyl alcohol solution was prepared as follows:

980 grams of water was heated to 160° F. at which temperature 19 gramsof Airvol 107 and 1 g of silanol modified polyvinyl alcohol (KurarayPoval R-2105) were added. The mixture of the two polymers and water washeated up to 195° F. until complete solubilization of the polyvinylalcohol occurred.

After the solution was cooled, 845 grams of CuI was added, and theresulting dispersion was ground in the Eiger mill, and then comminutedin the impingement mill until a bi-modal distribution of the dispersionwas achieved. The mode with the smaller particles was separated bycentrifugation to give a colloidal dispersion of CuI particles, with anaverage particle size of about 130 nanometers. No foam was observed inthe dispersion and when this dispersion was coated on the polyestersupport with Mayer rod #2, a thin conductive coating of no more than 0.2microns (by SEM analysis) was obtained.

A discrete conductive layer having no observable defects was formed,which layer also exhibited excellent adhesion to the polyester supportand was not removable by Scotch.sup.® tape, even after making severalcuts in the coating with a sharp knife. While no surface defects wereobserved, the coating was yellowish in color.

EXAMPLE 2

Half of the binder used in Example 1 was used in formulating the coatingdispersion of the present Example. Thus, 990 g of water, 9.5 g of Airvol107 and 0.5 grams of Kuraray R-2105 were used to prepare a resin bindersolution. 845 grams of CuI were added to the binder solution and wassubjected to a two-step grinding process as in Comparative Example 1 andExample 1. A useful bi-modal distribution was obtained after sevenpasses through the impingement mill. The mode with the smaller averageparticle size (average particle size of 160 nanometers) was separatedand the resulting dispersion was coated on a polyester support to give aconductive coating, which exhibited excellent adhesion despite usingonly half the amount of binder as in Comparative Example 1 andExample 1. When a coating was applied to a polyester support using MayerRod #2, a surface resistivity of 5.7×10⁷ ohms per square was observed.

EXAMPLE 3

A dispersion was prepared as in Example 2. Potassium iodide was added tothe dispersion, in accordance with the present invention, in the amountof 10 grams. This resulted in the following composition for thedispersion:

CuI - 845 grams

polyvinyl alcohol (Airvol 107) - 9.5 grams

silanol modified polyvinyl alcohol - 0.5 grams

KI - 10 grams

This dispersion was then subjected to a two-step grinding process as inExample 2 until a bi-modal distribution was achieved.

A colloidal dispersion with an average particle size of 120 nm wasseparated from the bi-modal dispersion to give a coating dispersion,which was a violet color.

When the resulting dispersion was deposited on a polyester support withMayer rod #2, a thin conductive coating (of about 0.15-0.2 microns bySEM) was obtained, which had all of the desirable characteristics forelectrostatic materials, i.e., it had a light transmittance in the UVregion above 52% and as high as 90% in the visible part of lightspectra, the adhesion of the coating layer to the polyester support wasexcellent, as well as its surface resistivity, which was measured atfrom 1 to 6 megahoms per square, which is a desirable conductivity rangefor high density electrostatic printing. When a Mayer rod #3 was used incoating a support with the dispersion, an increase in conductivity wassufficient to give a conductive CuI layer with a surface resistivity of10⁵ ohms per square immediately after coating and drying. The surfaceresistivity was 10⁴ per square after the material was maintained atambient conditions for 5 minutes or longer. A conductive coating layerof this conductivity is desirable for the preparation ofelectrophotographic materials.

EXAMPLE 4

16 kg of the dispersion of Example 2 were prepared at an averageparticle size of 164 nanometers, and was applied to a polyester supporton a pilot coater using a reverse roll station. Some difficulties wereobserved in wetting the polyester support and the applicator roll. Thecolloidal coating dispersion was accordingly adjusted by adding 180grams of 2% solution of silanol modified PVA and by the addition of 26grams of potassium iodide. The conductivity of the resulting liquiddispersion was measured and found to be 7 Siemens. The wettingproperties of this dispersion were found to be excellent and aconductive coating was obtained after drying the coating well in a dryerat 205° F. for 45 seconds.

The conductive ground plane was more than 52% transparent in the UVregion and more than 80% transparent in the visible spectrum, and nodefects were observed. The conductive coating demonstrated excellentadhesion and showed a surface resistivity of 0.5 to 3.0 megahoms persquare, which is a highly desirable range for electrostatic printing.

EXAMPLE 5

The final dispersion of Example 4 was circulated in a coating stationfor eight hours and several rolls of conductive material were made.After 3-4 hours of circulation, the surface resistivity of the dry CuIcoating started to increase to 10⁷ ohms per square. A small amount offresh KI was added and the surface resistivity was restored to theprevious value of 10⁶ ohms per square.

The conductivity of the dispersion after addition of KI was measured andfound to be 9-10 Siemens. The coating process continued and the surfaceresistivity of the conductive ground plane was maintained in a range of1 to 3 megahoms per square. The resulting high quality ground planeconductive coating was analyzed by TGA method and consisted of:

CuI - 92%

polyvinyl alcohol - 4.2%

Silanol modified polyvinyl alcohol - 2.3%

KI - 1.5% The total amount of solids in the colloidal dispersion used toproduce the above-described ground plane was about 7.9-8% by weight. Thecomposition of the colloidal dispersion, which was used to produce thehigh quality conductive ground plane was as follows:

water - 92%

polyvinyl alcohol - 0.34%

silanol modified PVA - 0.18%

CuI - 7.4%

KI - 0.12%

EXAMPLE 6

A conductive ground plane was prepared according to the conditions ofExample 5, and was overcoated with a dielectric coating, which was basedon a polystyrene resin with silica particles dispersed in a solution ofthis resin in the organic solvent. The resulting dielectric material wassubjected to relative humidity of 25, 50 and 75% at 50° C. for 5 days.After this treatment, the material was equilibrated for two hours atambient temperature and was printed on various Versatec electrostaticprinters to give clear, defect free images with a reflectance density ofabout 1.25-1.30 optical density units independent of RH value,demonstrating that this particular coating composition is independent ofrelative humidity, despite the fact that the conductive layer of theground plane is comprised of semiconductive CuI dispersed in a watersoluble binder.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made without departingfrom the spirit and scope thereof.

What is claimed is:
 1. An electrostatic imaging element comprised of anelectrostatic layer and a ground plane, said ground plane beingcomprised of a support and a conductive coating layer thereon, whereinthe conductive coating layer is comprised of ohmic bridged cuprousiodide particles and a binder resin, with an ohmic bridging electrolytecomprised of a sodium or potassium iodide, thiosulfate, bisulfate,bisulfite or dithionate being present in the conductive coating layer tothereby provide ohmic bridging between the cuprous iodide particles. 2.The electrostatic imaging element of claim 1, wherein an ohmic bridgingelectrolyte comprised of potassium iodide is present in the conductivecoating layer.
 3. An electrophotographic imaging element comprised of aphotosensitive layer and a ground plane, said ground plane beingcomprised of a support and a conductive coating layer thereon, whereinthe conductive coating layer is comprised of ohmic bridged cuprousiodide particles and a binder resin, with an ohmic bridging electrolytecomprised of a sodium or potassium iodide, thiosulfate, bisulfate,bisulfite or dithionate being present in the conductive coating layer tothereby provide ohmic bridging between the cuprous iodide particles. 4.The electrophotographic imaging element of claim 3, wherein an ohmicbridging electrolyte comprised of potassium iodide is present in theconductive coating layer.